The present invention relates to the insertion and removal of a circuit device into and from a bus network. It is particularly, but not exclusively, concerned with the case where that circuit device is a printed circuit board.
It is well known to interconnect circuit devices by means of a bus network which enables the devices to signal one to the other. FIG. 1 of the accompanying drawings shows a schematic block diagram of such an arrangement, in which a plurality of circuit devices 2 are interconnected with each other and are connected to a bus controller via a bus. That bus comprises an address and/or data bus (communication bus) 5, a control bus 4, and power buses 3,7 connected to a power source 6. One of the circuit devices 2 is a master, which controls the other circuit devices 2 which act as slaves. When the master circuit device 2 wishes to signal to one of the slave circuit devices 2, the master circuit device signals to that slave, and to the bus controller, via the control bus 4, and then data can be passed between master and slave along the address and data bus 5 as indicated by arrow A. The bus controller 1 ensures that the signalling is controlled in the desired way.
However, it sometimes occurs that there is a failure in one of the components of the circuit, and then the appropriate circuit device 2 must be removed and replaced. Where it is the master circuit device that is removed, there is normally no problem since the removal of that device effectively stops the network operation. However, when a slave circuit device is to be removed, the situation is more complicated. It is normally not desired to shut-down the network completely before removal of any particular slave circuit device, and therefore the faulty circuit device must be removed while the system remains under power. If one of the circuit devices 2, e.g. that indicated by arrow B is removed, then it is found that there is a risk that transient signals generated when it is removed affect data which is being transmitted between the master and another of the slave devices. The present invention, therefore, is concerned with trying to overcome this problem, while permitting insertion or withdrawal of a circuit device when the system is on-line (i.e. without disconnecting the power source) rather than by complete shut-down of the network.
U.S. Pat. No. 4,079,440 proposes a system in which each circuit device is mounted on a printed circuit board, and the board has at least two connector plugs for connecting it to the power supply, with those plugs being of different lengths. Therefore, during insertion and removal, the time of contact with the power line of those two connector plugs is different, and this difference can be used to decrease fluctuations of the power source voltage during insertion and removal. However, this is only a limited solution to the problem, since it does not resolve all the causes of faulty signalling.
U.S. Pat. No. 4,200,865 discloses an arrangement in which the circuit devices are mounted on a printed circuit board for insertion into a bus network, and the circuit board is designed so that the input to or the output from the board can be locked in response to the presence of two conditions, namely the application of a request for removal or insertion, and the fact that the circuit device is not signalling to any other device. In this system, the circuit components of the circuit device are connected to an address bus via an output driver, and the aim of the invention of this patent is to lock that bus driver to prevent the bus line from being electrically disturbed.
However, it has been discovered that the arrangement of U.S. Pat. No. 4,200,865 has a problem. At the time the circuit device is inserted or removed, the power connection to the output driver is disconnected, and therefore its locking is no longer stable. As a result, during transient conditions at the time of insertion or removal of the circuit device, the output driver may become unlocked and, depending on the electronic conditions present elsewhere in the circuit device, the circuit device may again be free to transmit to the bus network, thereby causing faults within the signalling in that network.